[workchat] reintegrate into main (#215627)

## Summary

~**DO NOT MERGE:** depends on
https://github.com/elastic/kibana/issues/213468~

This PR reintegrates the work from the `workchat_m1` branch into `main`:

- introduces a 4th solution type, `chat`, that will be used for the
*WorkChat* project type.
- edit things in various platform code to introduce/handle that new
project type
- add plugins and packages for the workchat app. 

### To AppEx reviewers:

File change count is scary, but you can safely ignore anything from
`xpack/solutions/chat` (given it's solution code), and focus on your
owned changes, which are way more reasonable

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Joe McElroy <joseph.mcelroy@elastic.co>
Co-authored-by: Rodney Norris <rodney.norris@elastic.co>
Co-authored-by: Jedr Blaszyk <jedrazb@gmail.com>
Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
Co-authored-by: Meghan Murphy <meghan.murphy@elastic.co>

packages/kbn-optimizer/limits.yml

@@ -199,3 +199,7 @@ pageLoadAssetSize:
   visTypeXy: 46868
   visualizations: 41000
   watcher: 43598
+  wciExternalServer: 35000
+  wciIndexSource: 40000
+  wciSalesforce: 25000
+  workchatApp: 25000